U.S. Pat. No. Re. 27,617 (Olsen) teaches a process of making a louvered light control film by skiving a billet of altercating layers of plastic having relatively lower and relatively higher optical densities. Upon skiving the billet, the pigmented layers serve as louver elements, which, as illustrated in the patent, may extend orthogonally to the resulting louvered plastic film. U.S. Pat. No. 3,707,416 (Stevens) teaches a process whereby the louver elements may be canted with respect to the surface of the louvered plastic film to provide a film which transmits light in a direction other than perpendicular to the surface of the film. U.S. Pat. No. 3,919,559 (Stevens) teaches a process for attaining a gradual change in the angle of cant of successive louver elements.
Among the uses for such louvered plastic films are lenses and goggles as shown in U.S. Pat. No. 3,791,722 (Ahlberg et al.), to be worn where high levels of illumination or glare are encountered. The film may also be used for transparent covering for a backlighted instrument panel, such as the instrument panel of an automobile, to minimize reflections from the windshield. A louvered plastic film may also be used to give a black and white photographic negative the appearance of a positive made from the negative as taught in U.S. Pat. No. 3,653,138 (Cooper).
U.S. Pat. No. 3,922,440 (Wegwerth et al.) points out that because louvered plastic films to are thin sheet materials: (1) they are not by themselves capable of structurally withstanding extreme stresses and (2) they are subject to distortion from physical stress and temperatures" (col. 1, lines 19-22). Furthermore, the skiving by which the louvered plastic films are produced results in irregular surfaces which seriously limits the optical quality of the film. Typically such films are, for practical purposes, translucent rather than transparent. Accordingly, as in Example 1 of that patent, the louvered plastic film usually is laminated under pressure between two clear plastic films of a material such as cellulose acetate butyrate, the material usually used in making louvered plastic films. Typically, the louvered plastic film is skived from the billet to a thickness between 0.1 and 0.4 mm and each of the outer plastic films has a thickness of between 0.1 and 0.3 mm. The ratio of the thickness of the skived film to the width of the clear regions will control the permitted view angle, with a greater ratio providing a narrower angle. Wegwerth's process of laminating louvered plastic films between two clear films requires an expensive press which is also expensive to operate. This is in part from the need to distribute heat uniformly and in part from the need to apply pressure with precision. Because the resulting laminates can not be larger than the platens of the press in which they are laminated, the press must be sufficiently large to produce the required size thus increasing the expense of the press.
U.S. Pat. Nos. 4,764,410 (Grzywinski) and 4,766,023 (Lu) teach alterative to the Wegwerth method. These alternative methods include the steps of (1) coating the skived louvered plastic film with a solventless monomer composition which polymerizes to an adhesive state or a hard state, respectively, upon exposure to radiation, (2) overlaying the monomer composition with a plastic film, and (3) exposing the coating to radiation to polymerize the composition. After polymerization the plastic liner which was placed over the monomer composition may be left in place to serve as protection for the louvered plastic film, or may be removed, leaving the polymerized composition exposed.
Such films are used for various purposes. One common use is to prevent light from automobile control panels from reaching the windshield and causing distracting and dangerous reflections at night. Another use is to cover the screen of a CRT or other display to prevent persons other than the operator from reading data displayed thereon.
A problem that is common to all of the louvered films described above, arises from the difference between the clear and dark layers. Typically the clear and dark layers are formed of the same material. A preferred material is cellulose acetate butyrate (CAB), although other materials may be used. The louvers are, however, rendered dark by the inclusion of very fine particles of another material. A preferred material is carbon black. If carbon black is used these particles have an average diameter of less than 0.1 .mu.m. Thus they are much smaller than the wavelength of the light.
In spite of the small size of these particles, and, in fact, in part because of that size, the presence of the particles causes the index of refraction of the composite to be different from that of the plastic alone. Since the index of refraction of the clear and dark layers are different, light is reflected at the interface between the two. The effect of this reflection is the creation of "ghost" images. The percentage of the incident light that is reflected increases with increasing angle of incidence and increasing difference of index of refraction. For these purposes the angle of incidence is the angle between the ray of light and a normal to the interface between the clear and dark layers. As a result the ghost images of a typical film are most noticeable at angles between 5.degree. and 25.degree. from the axis of the louvers. Such ghost images are aesthetically displeasing, at best. Furthermore, if the film is to be used to cover a CRT screen or other display, the ghost images can cause misinterpretation of data and significantly contribute to operator fatigue.
One approach to elimination of the ghosting problem is to provide a matte finish on the interface between the clear and dark layers. This tends to eliminate distinct ghost images, but does not reduce the total amount of light reflected. Thus the ghost image is replaced by a blurred, but clearly visible, glow. In addition the creation of such a matte surface is very difficult in the currently preferred coextrusion processes wherein the clear and dark layers are extruded together in a single process.
An alternative approach is to reduce the amount of carbon black incorporated into the layers forming the louvers. In this case, the difference in index of refraction between the clear and dark layers is less than in the previously described film and thus reflections are reduced. However, if the louvers remain the same width as those with higher optical density, they will no longer meet opacity requirements. Thus, such films may not be used as privacy screens. Alternatively the louvers may be made wider to meet opacity requirements. This, however, will reduce the on-axis transmission through the louvered film to unacceptably low levels and/or make the louvers individually visible.